Petrochemistry and isotope chemistry of Early Palaeogene basalts forming the dipping reflector sequence southwest of the Rockall Plateau

A transect across the passive margin SW of Rockall Plateau was drilled during DSDP Leg 81. Site 555 was located on continental crust of the Rockall Plateau. Sites 552 and 553 were located over the wedge of dipping reflectors, and Site 554 drilled the outer high at the transition from the thick wedge of dipping reflectors to normal oceanic crust. New trace-element and Pb-isotope data are presented which characterize three basalt types: (1) Dipping reflector basalts resemble depleted N-type MORBs, strongly depleted in light rare earth elements (LREEs) and other incompatible elements; they have been contaminated by U-depleted continental crust, or alternatively derived from depleted subcontinental lithosphere. (2) Outer high basalts are less strongly depleted N-type MORB and show no evidence of crustal contamination. (3) Site 552 basalts postdate the onset of subsidence of SW Rockall margin; they are relatively evolved N-type MORB derived from a source similar to that of the outer high basalts but show evidence of crustal contamination. Genesis of the dipping reflector basalts is best explained by a stretching/rifting model, not the ‘subaerial seafloor spreading’ model.